At present, international standard video coding schemes such as MPEG and ITU-T H.26x are utilized in a variety of applications in the fields of broadcasting, communications and packages such as digital broadcasting (satellite, terrestrial, cable), DVDs, video CDs, the Internet and mobile communications.
The need is growing for reusing video contents compressed by these coding schemes in a variety of platforms under conditions different in supporting coding schemes, transmission bit rates, spatial resolutions (frame sizes), temporal resolutions (frame rates). With this as a background, intensive research and development of video transcoding technology has been conducted.
In particular, the standard video coding schemes such as MPEG and ITU-T H.26x share the syntax of the coded data to some extent because they are consistently based on the signal redundancy reduction in the time-axis direction according to motion compensation (called MC from now on) and the signal redundancy reduction in the spatial direction according to discrete cosine transform (called DCT from now on). Considering conversions using them at a bit stream level, the transcoding technology that reduces computational load has been studied as to the standard video coding schemes.
Above all, converting MPEG-2 videos with an NTSC resolution (704×480 pixels per frame and 30 frames/sec) to lower frame rate MPEG-4 or H.263 videos with an SIF (352×240 pixels) becomes a hot topic because of an increasing demand for making effective use of existing MPEG-2 contents (refer to Wang Xing Guo, Zheng Wei Guo, and Ishfaq Ahmad, “MPEG-2 To MPEG-4 Transcoding”, Workshop and Exhibition on MPEG-4 (WEMP) 2001, for example).
In such video transcoding technology, it is important to convert motion vectors in an MPEG-2 video stream input to a transcoder into motion vectors usable in the MPEG-4 coding.
Since divide-by-two resolution conversion is made in both the vertical and horizontal directions, four macroblocks in an input MPEG-2 stream exactly correspond to one macroblock of the MPEG-4 coding. The problem is a one that estimates a motion vector after the resolution conversion from four original motion vectors at the maximum, and a variety of studies and reports have been made about it up to now. For example, B. Shen et al., “Adaptive Motion-Vector Resampling for Compressed Video Downsampling”, IEEE Transactions on Circuits And Systems for Video Technology, vol. 9, no. 6, September 1999 discloses a technique of calculating the weighted average of the four motion vectors by assigning larger weights to motion vectors with a larger prediction residue in the input compression data.
It is so to speak a method of deciding the values of the motion vectors to be converted by using the prediction residual signal in the input compression data as a reference and in accordance with its activity values. A variation of this method is reported by M. R. Hashemi, et al., “Compressed Domain Motion Vector Resampling for Downscaling of MPEG Video”, IEEE International Conference on Image Processing, Kobe, Japan, October 1999.
With the foregoing configuration, the conventional video data converting method has problems in that its application is limited to a case where the motion vector to be converted is only one, and that it does not ensure the optimum conditions in terms of coding performance of the motion vector after the conversion.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a video data converter and video data converting method capable of converting motion vectors considering the impact of the coding performance within a second video coding scheme.